Rulings
If there is somehow no chosen color, Heraldic Banner's static ability has no effect, and its mana ability produces no mana if you activate it. Colorless creatures won't get +1/+0, and you won't add {C}.
(wotc • 2019-10-04)
You must choose white, blue, black, red, or green for Heraldic Banner's ability. You can't choose "multicolor," "gold," or "colorless."
(wotc • 2019-10-04)
Rulings
If there is somehow no chosen color, Heraldic Banner's static ability has no effect, and its mana ability produces no mana if you activate it. Colorless creatures won't get +1/+0, and you won't add {C}.
(wotc • 2019-10-04)
You must choose white, blue, black, red, or green for Heraldic Banner's ability. You can't choose "multicolor," "gold," or "colorless."
(wotc • 2019-10-04)